The present invention relates to insulation construction generally, and more particularly to electrical insulation that are suitable for use in conjunction with toroidal transformers.
Toroidal transformers have included insulation in the form of arcuate (semi-toroidal) tubes which surround a portion of the core (and bobbin if used) of the transformer. U.S. Pat. No. 4,665,952 to Macemon, et al. (the disclosure of which is hereby incorporated herein) describes a toroidal transformer having two such tubes. Together the tubes form approximately 330.degree. of a toroid and serve to insulate the low voltage windings that surround the insulation tube from the core.
Each insulating tube described in U.S. Pat. No. 4,665,952 is formed from electrical kraft insulating paper that is wet molded into the form of the tube. As is well known in the art, electrical kraft paper performs an insulating function by virtue of oil-impregnation. Because of the excellent insulation properties of oil-impregnated electrical kraft paper, electrical kraft paper is widely used in transformer construction. To ensure proper oil-impregnation of the kraft paper, and thereby ensure that the insulation will have the appropriate dielectric strength, transformers are typically heated and evacuated after assembly to remove the moisture and air from the kraft paper. Oil is introduced into the evacuated transformer filling the microscopic spaces in the electrical kraft paper that were occupied by air before the evacuation.
Even though electrical kraft paper can be successfully molded by wetting the paper, the process is expensive. The wet molding method, as is well known, requires a time-consuming drying step before the molded paper insulation can be used. Efforts to speed up the drying process by applying heat have been only partially successful, since very high temperatures, which would cause the water to vaporize into steam, would damage the paper. Accordingly, only relatively low temperatures can be used resulting in long drying times.
In addition to the cost disadvantage of the molded paper insulation, the insulation kraft paper tube has strength limitations relative to the heavy forces imposed upon the tube when heavy-gauge conductors are wound upon the tube. This required the use of a relatively close-fitting arbor placed internally in the tube to support the walls during winding as disclosed in U.S. Pat. No. 4,771,957 to Schlake, et al. To prevent the walls from collapsing during winding of the relatively heavy gauge conductor, it was necessary to have close dimensional correspondence between the electrical kraft paper tube and the internally positioned arbor. That dimensional requirement complicated manufacture of the electrical kraft paper tube since it added a manufacturing tolerance that was not easily met in a wet-molded paper product.
Electrical kraft paper tubes have also been made by forming a long strip of paper in a spiral with the edges of the paper being overlapped to form a continuous piece of insulation. The overlapped sections created raised portions on the surface of the tube which often resulted in cross-overs in the windings wrapped therearound. This reduced the transformer's efficiency.
Efforts to resolve some of the aforementioned problems have included substituting plastic materials for the oil-impregnated electrical kraft paper. However, the fixed dimensions of the relatively inelastic plastic tubes have been found deficient in that they generally are not capable of closely conforming to the actual configuration of the core. Accordingly various tubes of different dimensions must be inventoried to accommodate cores of different sizes.
Therefore, there is a need to provide insulation tubing that can withstand the electrical and mechanical stresses between the core and outer windings, while being capable of closely conforming to the configuration of the core.